Right. It might help you to know what it is.
Why is he mutated? Was it a result of eating trash that was contaminated?
You see how that ties into your story? You don't need to figure out all the details & you don't have to even show a lot, but having a general idea like that helps to give it more meaning. It doesn't have to be that idea, it just has to make sense to you. Maybe there's a news story on TV talking about it indirectly, low in the background, or just something about an attack a few towns away.
You don't have much time in your short so maybe you introduce the figure right at the beginning. Keep him in the shadows, making strange noises, maybe dragging a bloody dead body in the foreground, with the burger place very far in the background. Maybe the same song's being played on the dead person's phone. He drops the body behind something out of view, we hear him eating.
Then continue with what you have. Then when Shaun goes outside, tease that out a bit more. When he hears something, he goes to see what it is, maybe because he wants to make sure his bike or car is okay. After he checks his ride, he sees the blood trail, he follows it, it goes across the street, then he sees a severed hand on the asphalt. He yells. And the yell makes the figure notice him & come after him. He runs back to work.
You set it up, then keep building it up.